Engine fuel may be pumped out of a fuel tank by a lift pump. The lift pump propels fuel towards a fuel rail before being injected by fuel injectors. A check valve may be included between the lift pump and the fuel rail to maintain fuel rail pressure and prevent fuel in the fuel rail from flowing back towards the lift pump. Operation of the lift pump is typically feedback controlled by an engine controller based on outputs from a pressure sensor coupled in the fuel rail. The controller attempts to maintain the pressure in the fuel rail to a desired pressure by adjusting an amount of electrical power supplied to the lift pump based on a difference, or error, between the desired fuel pressure and a measured fuel pressure obtained from the pressure sensor.
Thus, the lift pump replaces fuel lost to injection in the fuel rail. As fuel injection rates decrease therefore, the fuel resupply demands of the fuel rail correspondingly decrease, and the controller reduces the electrical power supplied to the lift pump. Consequently, the energy demands of the lift pump may be substantially proportional to fuel injection rates. In some examples, such as during engine idle and/or deceleration fuel shut-off (DFSO), the amount of electrical power supplied to the lift pump may drop sufficiently low, such that it may be more energy efficient to operate the lift pump in a low fuel flow mode. In the low fuel flow mode, the lift pump is not continuously powered nor powered via a duty cycled voltage as it would be with pulse width modulation (PWM). Instead, the lift pump may remain off and then may only be powered on when needed. For example, U.S. Pat. No. 7,640,916 describes an approach where under low engine loads, the lift pump remains off, and is only powered on to refill an accumulator.
However, the inventors herein have recognized potential issues with such systems. As one example, there may be a delay between lift pump power adjustments and observed fuel rail pressure changes. That is, it may take an amount of time before changes in lift pump power are reflected in the fuel rail pressure (assuming a substantially constant fuel injection rate). For example, when powering on the lift pump, the lift pump will not begin to add pressure to the fuel rail until the pressure upstream of the check valve, positioned between the lift pump and the fuel rail, exceeds the pressure downstream of the check valve. Thus, when the lift pump is powered on, the lift pump may not immediately start adding pressure to the fuel rail. In such examples, if the lift pump is powered on when the fuel rail pressure decreases to a minimum threshold, the fuel rail pressure may continue to decrease below the minimum acceptable level while the lift pump builds pressure upstream of the check valve. Such lift pump delays, may therefore result in fuel rail pressure undershoots and/or overshoots, which may result in fueling errors that can lead to drivability and robustness issues.
As one example, the at least some of the issues described above may be at least partly addressed a method comprising maintaining a lift pump off that supplies fuel to a fuel rail, assuming that the lift pump is maintained off, predicting when a fuel rail pressure will decrease below a threshold based on fuel injection rates, and powering on the lift pump before the fuel rail pressure decreases below the threshold such that actual fuel rail pressures do not decrease below the threshold. By powering on the lift pump before the fuel rail pressure decreases below the threshold, fuel rail pressure undershoots may be reduced.
In another example, a method comprises predicting when a fuel rail pressure will decrease below a threshold, calculating a desired instance to power on a lift pump based on a lift pump delay period, where the desired instance precedes when the fuel rail pressure is predicted to decrease below the threshold, stepping up a voltage supplied to the lift pump from zero to a first level at the desired instance, and ramping up the voltage supplied to the lift pump from the first level after the desired instance.
In yet another example a system comprises a lift pump, a fuel line coupled to the lift pump and comprising a fuel rail, the fuel rail including one or more fuel injectors, the fuel line delivering fuel from the lift pump to the fuel injectors, a check valve positioned in the fuel line between the lift pump and the fuel rail for maintaining fuel pressure downstream of the check valve, between the check valve and the fuel injectors, and a controller in electrical communication with the lift pump, the controller including computer readable instructions stored in non-transitory memory for: while the lift pump is off, predicting a decay profile for the fuel pressure downstream of the check valve, determining an instance to power on the lift pump based on the decay profile and a delay period of the lift pump such that the fuel pressure downstream of the check valve does not decrease below a threshold, and powering on the lift pump at the determined instance, before the fuel pressure downstream of the check valve reaches the threshold.
In this way, fuel rail pressure undershoots may be reduced. Specifically, by predicting how long it will take a lift pump to begin adding pressure to a fuel rail and forecasting future fuel injection rates, lift pump activation can be scheduled to prevent the fuel rail pressure from decreasing to undesirably low levels. As such, the lift pump can be kept off, increasing fuel savings, and then can be powered on at the appropriate time to prevent losses in engine performance and torque delivery.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.